1. Field of the Invention
The present invention relates to electrical overcurrent protection devices, particularly electronic trip circuit breakers having a manually-controlled arc flash energy reduction system.
2. Description of Related Art
Overcurrent protection devices (OPDs) are used in electrical distribution systems to protect electrical conductors and equipment against the effects of short circuits, ground faults, and/or overloads (hereinafter “faults”). The OPDs in an electrical distribution system are often selectively coordinated so that the nearest OPD upstream to a fault will open and clear the fault before another further upstream OPD opens. Selective coordination of OPDs limits the number of distribution circuits that are de-energized by the operation of an OPD in response to a fault. However, selective coordination of OPDs may also result in added time delays that could allow more energy to be released during a fault than would have been released had the OPDs not been selectively coordinated.
The OPDs in an electrical distribution system can be circuit breakers having programmable electronic controllers for controlling the OPDs' trip settings. The programmable electronic trip controllers are known as electronic trip units and circuit breakers employing electronic trip units are known as electronic trip circuit breakers. Selective coordination among electronic trip circuit breakers is achieved by appropriately adjusting the trip settings of the electronic trip units.
Electronic trip circuit breakers may accommodate zone selective interlocking (ZSI). In a ZSI system, a communication system exists between an upstream circuit breaker and all circuit breakers immediately downstream from the upstream breaker. A protection “zone” is formed that extends to the line side of the downstream circuit breakers
The upstream circuit breaker uses a first set of settings (collectively referred to as a “first trip setting”) that provide fast interruption for faults located in its protection zone (i.e., faults occurring between the upstream circuit breaker and a downstream circuit breaker). The upstream circuit breaker uses a second set of settings (collectively referred to as a “second trip setting”) for faults located outside of its protection zone (e.g., faults occurring downstream of a downstream circuit breaker). The second trip setting operates more slowly than the first trip setting and is coordinated with a trip setting for the downstream circuit breaker. Because the first trip setting provides a faster interruption than the second trip setting, less energy may be released during a fault that occurs within the upstream circuit breaker's protection zone than a fault occurring outside of the zone.
The second trip setting provides selective coordination with the downstream circuit breakers and, therefore, has added time delays that may allow more energy to be released during a fault that occurs outside of the upstream circuit breaker's protection zone. Selective coordination between upstream and downstream breakers is achieved by adding time delays to the electronic trip unit of the upstream breaker to thereby give the downstream breaker time to interrupt the fault.
In a ZSI system, when a downstream circuit breaker detects a fault, it will send a restraint signal to the upstream circuit breaker. The upstream circuit breaker, upon seeing the restraint signal, will recognize that the fault is outside of its protection zone and begin to time out based on its second trip setting. In a first scenario, if the downstream circuit breaker operates properly, it will trip and clear the fault before the upstream circuit breaker times out. Further, the upstream circuit breaker will determine that the fault has stopped and will stop timing using its second trip setting and, thus, will not trip. In this first scenario, the downstream circuit breaker cleared the fault and minimal distribution lines were affected.
In a second scenario, if the downstream circuit breaker does not operate properly, the second trip setting on the upstream circuit breaker will time out and the upstream breaker will trip and clear the fault. Thus, the upstream circuit breaker acts as a back up circuit breaker to the downstream circuit breaker. In this second scenario, however, all distributions lines downstream from the tripped upstream circuit breaker are de-energized and additional fault energy was released.
In a third scenario, if the upstream circuit breaker detects a fault and does not receive a restraint signal from a downstream circuit breaker, the upstream breaker will recognize that the fault is within its protection zone and use its fast first trip setting, thereby minimizing the energy released during the fault.
One drawback of a ZSI system is that control wiring and conduit must be installed between upstream and downstream circuit breakers, so that restraint signals can be monitored by the upstream circuit breaker.
A fault condition could cause an electrical arc that is harmful to nearby persons or property, due to the incident energy of the arc flash. It would be desirable to limit the duration of such an arc, and, therefore, the incident energy of the arc flash. It would further be desirable to limit the incident energy of an arc flash while also minimizing or eliminating the need to install wiring and conduit between OPDs as required for a ZSI system. One method of limiting the duration of an arc caused by a fault condition is by adjusting the trip settings of an electronic trip circuit breaker to lower and/or faster settings. However, selective coordination may be lost by such an adjustment. It would be desirable to provide an electronic trip circuit breaker having a convenient trip adjustment for temporarily adjusting trip settings to lower and/or faster settings.